Gas turbines are known to comprise the following elements: a compressor for compressing air; a combustor for producing a hot gas by burning fuel in the presence of the compressed air produced by the compressor; and a turbine for expanding the hot gas produced by the combustor. Gas turbines are known to emit undesirable oxides of nitrogen (NO.sub.x) and carbon monoxide (CO). One factor known to affect NO.sub.x emission is combustion temperature. The amount of NO.sub.x emitted is reduced as the combustion temperature is lowered. However, higher combustion temperatures are desirable to obtain higher efficiency and CO oxidation.
Two-stage combustion systems have been developed that provide efficient combustion and reduced NO.sub.x emissions. In a two-stage combustion system, diffusion combustion is performed at the first stage for obtaining ignition and flame stability. Premixed combustion is performed at the second stage to reduce NO.sub.x emissions.
The first stage, referred to hereinafter as the "pilot" stage, is normally a diffusion-type burner and is, therefore, a significant contributor of NO.sub.x emissions even though the percentage of fuel supplied to the pilot is comparatively quite small (often less than 10% of the total fuel supplied to the combustor). The pilot flame has thus been known to limit the amount of NO.sub.x reduction that could be achieved with this type of combustor.
Pending U.S. patent application Ser. No. 08/759,395, assigned to the same assignee hereunder, (the '395 application) discloses a typical prior art gas turbine combustor 100. As shown in FIG. 1 herein, the combustor 100 comprises a nozzle housing 6 having a nozzle housing base 5. A diffusion fuel pilot nozzle 1 having a pilot fuel injection port 4 extends through nozzle housing 6 and is attached to nozzle housing base 5. Main fuel nozzles 2 extend parallel to pilot nozzle 1 through nozzle housing 6 and are attached to nozzle housing base 5. Fuel inlets 16 provide fuel to main fuel nozzles 2.
A main combustion zone 9 is formed within liner 19. A pilot cone 20 projects from the vicinity of pilot fuel injection port 4 of pilot nozzle 1 and has a diverged end 22 adjacent to the main combustion zone 9. Pilot cone 20 has a linear profile 21 forming a pilot flame zone 23.
Compressed air 101 from compressor 50 flows between support ribs 7 through main fuel swirlers 8 into the main combustion zone 9. Each main fuel swirler 8 has a plurality of swirler vanes 80. Compressed air 12 enters pilot flame zone 23 through a set of stationary turning vanes 10 located inside pilot swirler 11. Compressed air 12 mixes with pilot fuel 30 within the pilot cone 20 and is carried into the pilot flame zone 23 where it combusts.
FIG. 2 shows an upstream view of combustor 100. As shown in FIG. 2, pilot nozzle 1 having pilot fuel injection port 4 is surrounded by a plurality of main fuel nozzles 2. A main fuel swirler 8, having a plurality of swirler vanes 80, surrounds each main fuel nozzle 2. The diverged end 22 of pilot cone 20 forms an annulus 18 with liner 19. Fuel/air mixture 103 flows through annulus 18 (out of the page) into main combustion zone 9 (not shown in FIG. 2).
It is known that gas turbine combustors such as those described in FIG. 1 emit oxides of nitrogen (NO.sub.x), carbon monoxide (CO), and other airborne pollutants. While gas turbine combustors such as the combustor disclosed in the '395 application have been developed to reduce these emissions, current environmental concerns demand even greater reductions.
It is known that increased pilot flame stability affects NO.sub.x and CO emissions by allowing the pilot fuel to be decreased. The linear profile pilot cones known in the art are somewhat effective in controlling pilot flame stability by shielding the pilot flame from the influx of high velocity main gases. These pilot cones also form an annulus that prevents the main flame from moving upstream of the flame zone (flashback). However, constricted pilot recirculation zones and vortex shedding at the diverged ends of these pilot cones are known to cause instability in the pilot flame.
Similarly, it is known that leaner fuel/air mixtures burn cooler and thus decrease NO.sub.x emissions. One known technique for providing a leaner fuel mixture is to create turbulence to homogenize the air and fuel as much as possible before combustion. However, the pilot cones known in the art do little to create this type of turbulence.
As fuel mixtures become leaner, however, pilot flame stability becomes more important. That is, for a gas turbine combustor to be self-sustaining, the pilot flame must remain stable even in the presence of very lean fuel/air mixtures.
Thus, there is a need in the art for pilot cones that reduce NO.sub.x and CO emissions from gas turbine combustors by providing increased pilot flame stability with leaner fuel/air mixtures.